The present invention relates generally to a coated paper product having high gloss and brightness and the method of manufacturing such a product. In particular, the invention relates to a process for manufacturing a coated paper product with a surface comparable to a cast coated surface, that may be used, for example, as the facing sheet of a pressure sensitive laminate. In addition to this intended use, the product of the present invention is suitable for a variety of other printing and converting operations such as metallizing, foil laminating and printing, security label applications and, specialty packaging as well as upscale gift wrap and labels.
Such paper products have in the past been produced almost exclusively by a cast coating process. During cast coating, gloss development relies on a replication of the mirror-like finish on a dryer roll, as the applied coating is dried. However, production rates for the cast coating process are considerably slower than the production of coated paper on a high speed papermachine. Thus it would be desirable and advantageous to develop a high speed coating process that could be used to produce a cast coated surface on paper. Examples of the cast coating process are disclosed in prior U.S. Pat. Nos. 4,241,143 and 4,301,210.
Another method for producing high gloss paper is disclosed in U.S. Pat. No. 5,360,657. In this patent, a process is disclosed in which a thermoplastic polymeric latex having a second order transition temperature of at least 80 degrees C., and an average particle size smaller than 100 microns is applied to paper which is subsequently calendered to produce high gloss. Other methods for producing high gloss paper include the application of a glossy overprint varnish onto a previously coated substrate. However, in the latter case, the glossy surface produced is not generally useful for offset printing because of the excessive ink drying time required.
It is also known, as disclosed for example in PCT published application WO 98/20201, that a printing paper having high brightness and gloss can be manufactured by applying to paper a coating comprising at least 80 parts precipitated calcium carbonate and at least 5 parts of an acrylic styrene copolymer hollow sphere plastic pigment. The published application also notes that a finishing step using a calender is required to achieve the gloss development, but the method of calendering is deemed to be not restrictive. Likewise, in an article entitled "Lightweight Coated Magazine Papers," published in the Jul. 5, 1976 issue of the magazine PAPER, Vol. 186, No. 1, at pages 35-38, a relationship between calendering and the use of plastic pigments in coatings is disclosed. The article notes that polymers such as polystyrene are thermoplastic and pressure sensitive, and a pigment based on polystyrene will exhibit a high degree of calendering response.
These and other publications including an article entitled "Light Reflectance of Spherical Pigments in Paper Coatings," by J. Borch and P. Lepoutre, published in TAPPI, February 1978, Vol. 61, No. 2, at pages 45-48; an article entitled "Plastic Pigments in Paper Coatings," by B. Aluice and P. Lepoutre, published in TAPPI, May 1980, Vol. 63, No. 5, at pages 49-53; and an article entitled "Hollow-Sphere Polymer Pigment in Paper Coating," by J. E. Young, published in TAPPI, May 1985, Vol. 68, No. 5, at pages 102-105, all recognize the use of polymer pigments in paper coatings, but none of these publications disclose the unique combination of coating formulation and finishing conditions disclosed herein.